The principle of electromagnetic force compensation has a wide field of application in the many diverse kinds of balances that are used in commerce, in industry, and in laboratories. It is a particular strength of this principle that it allows balances of enormous measuring accuracy to be realized. With an analytical balance that functions according to the principle of electromagnetic force compensation, it is for example possible to measure a weighing load of 100 grams with a measurement resolution of 0.01 milligrams, i.e. with an accuracy of one part in ten million.
The generic type of balance or weighing cell to which the present invention belongs has a stationary base part and a load receiver constrained to the base part so as to allow guided movement of the load receiver relative to the base part, wherein the load receiver serves to receive the weight force of a weighing load. Mounted on the stationary base part is a permanent magnet system with an air gap. A coil in which a compensation current flows is connected to the load receiver by a force-transmitting mechanism and is suspended in the air gap with guided mobility. An optoelectronic position sensor, whose sensor signal is representative of the displacement of the interlinked movable parts of the balance from a zero position which occurs as a result of placing a load on the load receiver, typically includes a light emitter and a light receiver which are mounted on the base part with an interstitial space between them, and further includes a shutter vane which extends through the interstice and participates in the displacement travel of the movable parts. The signal of the position sensor is sent to a closed-loop controller which, in response, regulates the compensation current in such a way that the shutter vane and the movable parts of the balance that are connected to it are returned to their zero position by the electromagnetic force that is acting between the coil and the permanent magnet. In other words, the function of the closed-loop regulation is to maintain equilibrium between the electromagnetic compensation force and the weighing load. According to the laws of electromagnetism, the strength of the coil current and the resultant force are proportional to each other, and the weight of a weighing load placed on the load receiver can therefore be determined by measuring the coil current.
Within the area delineated by the foregoing description, the present invention focuses on the optoelectronic position sensor. The primary requirement that the position sensor of an electromagnetic compensation balance has to meet is that the zero point, i.e. the position of the shutter vane relative to the stationary base part when the sensor signal crosses the zero threshold between negative and positive values, needs to be maintained with the highest degree of accuracy and reproducibility. In addition, the relationship between the sensor signal and the deflection of the shutter vane should as much as possible be linear and reproducible. These requirements need to be met in particular within a given range of ambient temperature and humidity.
In an optoelectronic position sensor according to U.S. Pat. No. 3,805,907, the light source consists of a light-emitting diode, and the light receiver is composed of two phototransistors in a differential arrangement. The phototransistors are arranged diametrically opposite to each other on the face of a carrier disk that is rotatably mounted on the stationary base frame of the balance. By turning the carrier disk the sensitivity, i.e. the magnitude of the sensor signal in relation to the deflection of the shutter vane, can be adjusted. However, this relationship is by no means linear. Strictly speaking, it can therefore not be expressed as a proportionality factor except in the immediate area of the zero point (where it is defined as the slope of the sensitivity curve at the zero point).
In a further optoelectronic position sensor, which is described in U.S. Pat. No. 4,825,968, the light emitter and the light receiver of the position sensor are arranged facing each other across a central cutout in the cover plate of the permanent magnet system. The shutter vane, in this case a light barrier with a slot-shaped aperture which is attached to a balance beam that also carries the compensation coil, reaches upwards into the cutout as a movable light gate between the light emitter and the light receiver. In this arrangement, the reference for the zero position, i.e. the position of the shutter vane where the sensor signal crosses the zero threshold between negative and positive values, is not the supporting base frame of the balance, but the cover plate of the magnet. Due to the different coefficients of thermal expansion in the base frame of the balance and in the permanent magnet arrangement, the zero point reference for the measurements taken by the balance can therefore be subject to a temperature drift.
At a stage in the manufacturing process prior to the actual temperature compensation, the weighing cell is exposed to a series of large temperature swings, i.e. to an aging process, which has the purpose to stabilize the hysteresis loops of the sensitivity and of the zero point and to minimize the extent of further aging. The phenomena of hysteresis and aging can be caused by microscopic displacements in the connecting zones between weighing cell components with unequal thermal expansion.
It is therefore desirable to reduce the number of the components and connecting areas involved, to match the expansion properties of the individual components to each other, and thus to shorten or to entirely save the time required for the aforementioned aging process.
It is therefore the object of the present invention to create a position sensor for a balance that is based on the principle of electromagnetic force compensation, wherein the position sensor surpasses the existing state of the art in meeting the aforementioned primary requirements within a given range of ambient temperature and atmospheric humidity. The first requirement concerns the accuracy and reproducibility of maintaining the zero position which in the present context means the position of the shutter vane relative to the stationary base part when the sensor signal crosses the zero threshold between negative and positive values. In particular, a solution that meets the objective of the invention should reduce the aforementioned temperature hysteresis of the zero point of the measurement scale as much as possible and eliminate the need to perform more aging cycles. Furthermore, the relationship between the sensor signal and the deflection of the shutter vane should as much as possible be linear and reproducible. A further aim is to solve the inventive task in a way which optimally meets the technical conditions imposed by the manufacturing process.